Imo the best way to utilize the Essenial Phantom (until you get Caedebald's Blackbow like @SenSx said, that's an awesome combo) is to give it Concelhaut's Draining Touch as a single handed weapon or with Kapana Taga (reason for Kapana Taga see below). At the same time I let Aloth cast Miasma of Dull-Mindedness onto enemies who will get attacked by the phantom. Bonus points if you have somebody in the party who uses clubs + modal (for me it's often Edér with Shattered Vengeance or Kapana Taga).
Club modal + Miasma results in -65(!) Will defense. Maybe Aloth follows up with a mass Confuse and somebody adds a Shaken/Frighten/Terrify effect (Xoti, Aloth) and some -10 defense to all (see Shining Beacon or Ben Fidel's Neck) and enemies' Will defense is at -95 without too much effort. Should you use some party member with maxed Survival and Ngati's Tusk (Hunter of Hunters enchantment) enemies' will is at -105.
The reasons why this is a great combination (besides that fact that debuffing Will profits other party members as well) :
Concelhaut's Draining Touch doesn't go away on a Phantom. It stays as long as the phantom will last.
it targets Will instead of Deflection. Since Will is so easily debuffed the Phantom will land lots of crits
as single weapon the phantom will get +12 ACC
Draining Touch has very high base damage. It works very well with crits (likely overpenetration + crit damage bonus)
It weakens the enemy (prevents proper healing and lowers max health as well as fortitude)
Since it weakens on hit it works with the Helm of the White Void (+10 ACC, also works with the phantom)
it drains health which makes the phantom a lot sturdier
Now you can give Aloth items with +engagement (like Kapana Taga: +2 - or a small shield: +1) and terrify enemies with Aloth (Ryngrim's spells). The enemies will not only get their Will debuffed but also disengage constantly. If the phantom has engagement from items it will use disengagement attacks which have a big dmg bonus - which again works very well with the Draining Touch's high base damage. I usually also give the phantom boots of speed. That way it will close gaps to disengaged enemies very quickly again, re-engaging.
It is a very potent setup rel. early in the game: a summon that is sturdy and deals tremendous damage while it also causes weakened.
In addition one can wear items that trigger AoE damage on knockout or when damaged: White Witch Mask (Repulsive Visage when blooded), Effigy's Husk, Mantle of the Seven Bolts and such. They also work when the phantom gets killed (not if the duration is up and it simply vanishes though). And it the effects are limited to 1/rest it doesn't matter as long as Aloth doesn't use it up: the phantom will get a "fresh" copy of the item with 1 use/rest every time it gets summoned. You can also give Voidwheel to the phantom so it damages itself with every attack and eventually kills itself, triggering those effects deliberately. Or the Great Sword Effort: it does a Full Attack to all enemy bystanders if the phantom dies.
Other nice things to do with the phantom:
give it Essence Interrupter - the effect to summon a friendly creature after the enemy is dead also works with the phantom and its copy of the bow
Willbreaker or Blade of the Endless Paths (or other weapon which put limited "stacks" of debuffs on enemies on hit) : Since you will have two weapons (1 original , 1 copy) you will put two separate stacks of debuffs on enemies. 2 Willbreaker for example will put 2 stacks of will debuff on enemies, lifting the max of -15 Will to 2*-15 Will = -30 Will. BotEP will be able to debiff deflection by -40 instead of -20 only and so on. Another candidate is Ball and Chain with the Action Speed debuff for example.
Citzal's Spirit Lance
high deflection gear (armor such as Nomad's Brigandine, bracers, cloak) and Whispers of the Endless Paths with Offensive Parry - phantom will daze enemies if they miss and deal some damage with parries
Cast Salvation of Time on the phantom: it prolongs its summon duration
The Red Hand - pretty awesome if its enchanted with Double Tab ang you're fighting against vessels
I'm sure there's more cool stuff you can do with it. That's what I remembered from the top of my head.